Beyond the Vegard's law: solid mixing excess volume and thermodynamic potentials prediction, from end-members

A method has been developed, herein presented, to model binary solid solutions' volume, enthalpy and Gibbs energy using the energy state functions, E(V,S), of the end-members only. The E(V,S)s are expanded around an unknown mixing volume, VMix, and the fundamental equilibrium equation −(∂E/∂V)S=P is used to determine VMix. VMix allows us to model enthalpy, straightforwardly. The same argument holds using Helmholtz energy, F(V,T), in place of E(V,S), and the equilibrium equation becomes −(∂F/∂V)T=P. One can readily determine the Gibbs free energy, too. The method presented remarkably simplifies computing of solid mixings' thermodynamic properties and makes it possible to preserve crystal structure symmetry that would undergo reduction because of the introduction of disordered super-cells.